The present invention relates to a container for storage of radioactive waste and a method for producing the container.
Containers or vaults for storing radioactive waste must be able to isolate the radioactive waste from the environment for very long periods of time, for example, hundreds and thousands of years. Although the containers will usually remain undisturbed while being stored, there are periods when the vault or container must be moved and handled, for example, for placing into storage or moving to a different location, and is thus subject to accidental damage. Therefore, these containers must be able to withstand without breaking substantial shock loading caused by mishandling or dropping. Furthermore, the container must be able to withstand impact of heavy objects in case the building or structure in which the containers are stored collapses due to earthquakes or other external destructive forces. Moreover, it must also be ensured that the deliberate attempt to destroy a container cannot lead to a destruction or breakage of the container and, in this regard, the container must be able to withstand severe ballistic load or blast loads of explosives.
Another consideration is that of radiation shielding. Ideally, the storage container should be able to shield the environment more or less completely from radiation Generated by the stored radioactive waste.
These two properties, structural integrity and radiation shielding, require a container wall material that can absorb radiation and impact and that has a high fracture toughness and ballistic or blast resistance.
A material that has both of these features is slurry-infiltrated fiber concrete (SIFCON), disclosed, for example, in Concrete International, pp. 44-47, Dec. 1984, and in "Preparation, Properties and Applications of Cement-based Composites Containing 5 to 20 Percent Steel Fiber", Steel Fiber Concrete US-Sweden Joint Seminar, ed. S. P. Shah and A Skarendahl, Elsevier Science Publishers, 1985. SIFCON is a relatively new composite material that consists of short steel or other metal fibers embedded in a cement matrix of Portland cement. Unlike conventional fiber-reinforced concrete in which fibers are added to a concrete mixture resulting in a volume percent range of metal of 0.5 to 1.5%, SIFCON allows for the introduction of a considerably higher volume percentage of metal fibers within the cement matrix. SIFCON is produced by preparing a bed of preplaced fibers in the range of 5 to 30% by volume. The resulting fiber bed is then infiltrated with a low viscosity cementitious slurry. The cured metal fiber/cement composite possesses very high flexural and compressive strength as well as toughness and ductility. It has been demonstrated that SIFCON is highly resistant to blast loads and ballistic and fragment penetration. With the proper selection of fibers, SIFCON will also provide substantial radiation protection.
A fiber-reinforced concrete container for the storage of radioactive waste is known and produced by a company called Sogefibre (1, Rue des Herons, Montigny-Le-Bretonneux, 78182 Saint Quentin-Yvelines, Cedex, France). A highly corrosion-resistant, non-crystalline metal fiber Fibraflex is used in the concrete mixture. The containers can be cylindrical or cubical. The containers are manufactured by molding from a premixed fiber/concrete mix the bottom and the sidewalls in a single step and by molding a separate lid that is to be placed on the container after introduction of the radioactive waste. The lid must then be connected to the upward edges of the sidewalls by sealing the gap between the sidewalls and the lid with a fiber-concrete mixture and curing. A drawback of these containers is the seal between the lid and the container because the so-called cold joint between sidewalls and lid represents a discontinuity of the concrete enclosure that is prone to rupture or break when the container is subjected to impact or stress.
A further disadvantage of the known container is that fibers and the cementitious mixture are mixed to form a slurry that is then poured into the mold. By mixing fiber and the cementitious mixture, the amount of fibers that can be introduced into the concrete slurry is limited in order to maintain flowability of the concrete mix. In general, the amount of fibers is limited to 2 to 3% by volume. This also results in a much reduced reinforcement since the amount of fiber in the cement is substantially proportional to the attained reinforcement strength. Another often observed disadvantage of fiber/concrete premixtures is the non-uniform distribution of the fibers within the concrete: commonly, pockets of high density of fiber distribution are observed while other areas have a very low density of fiber distribution. This causes an uneven reinforcement with weak points.
It is therefore an object of the present invention to provide a container and a method for producing such a container with which the aforementioned disadvantage of having a weak point in the concrete enclosure is eliminated.